Optical fiber transducer

ABSTRACT

A measurand is detected by a sensor element forming part of a transducer unit which is filled to an optical fiber or fiber capable such that the fiber is bent or curved to an abnormal level which introduces a measurable loss in the transmission characteristics of the fiber. The transducer unit has three spaced clamp members arranged to be clamped along the length of the fiber. The sensor element is fitted between clamp members and the fiber is guided over a bar located between clamp members.

FIELD OF THE INVENTION

This invention relates to detection of a measurand utilising atransducer unit in association with an optical fibre or optical fibrecable.

According to a first aspect the present invention provides a transducerunit adapted to be fitted to an optical fibre, the transducer unitcomprising a support structure carrying an assembly of clamp membersmutually spaced from one another in a generally linear direction, theclamp members being adapted and arranged to be secured individually toan optical fibre extending in said direction, obstructive means disposedin the path of the optical fibres between first and second clamp membersof the assembly for pre-bending the fibre from its lenear path when thefibre is clamped, a sensor element which is sensitive to a particularmeasurand disposed between at least one of the first and second clampmembers and another clamp member of the assembly, the sensor elementbeing such that in operation exposure to the particular measurand causesa dimensional change of the sensor element in said linear direction, thearrangement of the transducer unit being such that the magnitude of thebend in the fibre is enlarged to an abnormal level which introduces ameasurable loss in the transmission characteristics of the fibre whenthe sensor element is exposed to the particular measurand.

SUMMARY OF THE INVENTION

The sensor element may be of the type which dimensionally enlarges ordimensionally contracts in said linear direction. Such sensor elementsmay be made of hydrogel material or shape memory alloy material.Dimensionally contracting sensor elements are disposed between the firstand second clamp members.

The sensor element is preferably made of a material where thedimensional change is reversible on removal from exposure to theparticular measurand. Such sensor elements are preferably secured ateach end to at least one of the first and second clamp members and tothe said another clamp member of the assembly.

The sensor element may be in the form of a hydrogel plug for sensingaqueous environments such as the presence of water or water vapour or pHvalue. It may be a rubber plug for sensing hydrocarbon presence. It maybe a swellable wax plug for sensing heat or it may be ashape-memory-alloy element such as a spring for sensing heat. It will ofcourse be recognised that the foregoing materials are given only by wayof example of the many materials which are suitable to form the sensorelements.

In one embodiment the assembly of clamp members are three in number andhave limited movement with respect to the support structure and arearranged so as to be mutually separated by respective distances each ofwhich is less than the length of the corresponding fibre portion whenthe clamp members are secured to be fibre. The sensor element isdisposed between the second and third clamp members. In this way whenthe sensor element is of the type which becomes enlarged on exposure tothe particular measurand it causes movement of the second or centralclamp member without being restrained by the presence of a comparativelyrigid portion of fibre between the first and second clamp members.

It is preferred that a resilient biasing element is located between thefirst and second clamp members for restoring the position of the centralor second clamp member after activation of the sensor element and in theabsence of the particular measurand.

The optical fibre is preferably a single mode fibre or a bundle ofsingle mode fibres forming a fibre cable. In this connection it will beunderstood that the integrity of the optical fibre does not require tobe compromised by use of the transducer unit in that there is nostripping of fibre cladding or overcovering or the like.

The condition of the optical fibre and the transducer unit associatedtherewith may be interrogated by transmission of a signal along thelength of the fibre and which is detected at the fibre output and by apower threshold detection unit. Alternatively a detection system whichuses optical time domain reflectometry (OTDR) may be used to locate theposition of a transducer unit incorporating an abnormally bent fibre.This is of particular value where several transducer units are fittedalong the length of the optical fibre. Several different types of sensorelements can be used in transducer units along the length of the samecable in order to detect different measurands. The loss characteristicscan be tuned so that different sensor types have different losssignatures on activation thereby enabling detection of differentmeasurands to be prioritised.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described withreference to the accompanying schematic drawings, in which:

FIG. 1 is a sectional side view of a transducer unit in accordance withthe present invention when fitted to an optical fibre;

FIG. 2 is similar to FIG. 1 but with components removed in the interestsof clarity; and

FIG. 3 is a plan view of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The transducer unit 10 illustrated in the drawings has a box-likesupport structure which is capable of being fitted to an optical fibre11 with minimal disturbance of the fibre. Unit 10 has a base portion 9which internally houses a set or assembly of three clamp members 12, 13,14 which are mutually spaced from one another in a generally lineardirection, being the direction of the fibre 11. The clamp members 12,13, 14 respectively include releasable clamp elements 12A, 13A, 14Awhich can be retrofitted to clamp against the outer surface of the fibre11. Extending transversely with respect to the fibre direction, andbetween clamp members 13 and 14, there is an obstructive member 15 whichprojects into the path of the fibre 11 for pre-bending the fibre out ofits strictly linear path. The obstructive member 15 conveniently is inthe form of a rod secured to the walls of the base 9.

Clamp member 12 is capable of limited movement towards clamp member 13and the latter is also capable of limited movement towards clamp member14. Clamp member 14, however, is effectively rigidly secured. Anoverfill feed wedge 16 is provided as part of a lid portion 8 of theunit 10 to adjust the position of clamp member 12 after it is clamped tothe fibre 11 so as to introduce an excess of fibre between clamp members12 and 13 as a result of which the fibre does not extend strictlylinearly in that portion.

A sensor element 17 which is sensitive to a particular measurand anddimensionally enlarges as a consequence is disposed between the clampmembers 12 and 13. In the illustrated embodiment this sensor element isin the form of a coil spring made of shape memory alloy (eg nitinol). Asthe temperature of the environment surrounding the spring 17 rises abovea trigger threshold level the nitinol goes through a phase transitionand the spring attempts to return to its pre-determined shape which isof greater axial length than is illustrated in FIG. 1. During thisrecovery process the spring releases large amounts of energy and issufficient to move clamp member 13 in the direction of clamp member 14at least to a limited extent. This movement causes buckling of the fibreportion overlying the obstructive member 15 and this bucklingconstitutes an abnormal bend which introduces a measurable loss in thetransmission characteristics of the fibre 11 at the location of thetransducer unit 10.

Once the temperature of the spring sensor element 17 returns to normalthe sensor element 17 can be restored to its FIG. 1 condition and thisis achieved by the provision of a resilient biasing member 18 whichconveniently is in the form of a stainless steel coil spring locatedbetween the clamp elements 13 and 14. Coil springs 17 and 18conveniently surround rod elements which ensure that the springs retaintheir coil shape.

It will be understood that the fibre 11 is associated with a detectionsystem 19 which enables the presence of an abnormally bent fibre 11 tobe identified. Detection unit 19 is preferably an optical time domainreflectometry system which provides a pulse or series of pulses to thefibre 11 and detects pulse reflections from abnormally bent fibreportions and is thereby able to locate the position of the abnormallybent fibre portions.

It will be understood that the transducer unit 10 can be attachedanywhere along the length of an optical fibre or optical fibre cable. Itwill also be appreciated that the various clamp members may take anyindividual format and for example it may not be necessary for the unit10 to incorporate a lid. It could be an open housing. Furthermore theoverfill feed element 16 may itself form a clamp element so that it maybe integrated with clamp element 12A.

When the transducer unit 10 is fitted to the fibre 11 fibre portion 11Awhich extends between clamp elements 13A and 14A is deflected from itsstrictly linear condition by the obstructive member or rod 15.Accordingly when clamp member 13 is moved by the sensor element 17 inthe direction of the clamp member 14 there is minimal resistance tofurther bending of the fibre 11 imposed by the fibre itself because italready incorporates a bend. Likewise clamp member 12 is initiallyfitted and clamped to the fibre 11 but thereafter the overfill feedelement 16 is applied and this causes movement of the member 12 towardsclamp member 13 which initial movement causes a minor deflection of thefibre portion 11B which lies between clamp members 12 and 13.Accordingly when the sensor element 17 expands the fibre portion 11Bimposes no restraint on movement of the clamp member 13. Clamp member 12is effectively fixed as is clamp member 14.

It will be understood that the transducer unit disclosed herein iselectrically passive and immune to electromagnetic interference and isvirtually maintenance free and can be retrofitted to the fibre 11.

What is claimed is:
 1. A transducer unit adapted to be fitted to anoptical fibre, the transducer unit comprising a support structurecarrying an assembly of clamp members mutually spaced from one anotherin a generally linear direction, the clamp members being adapted andarranged to be secured individually to an optical fibre extending insaid direction, obstructive means disposed in the path of the opticalfibre between first and second clamp members of the assembly forpre-bending of the fibre from its linear path when the fibre is clamped,a sensor element which is sensitive to a particular measurand disposedbetween at least one of the first and second clamp members and anotherclamp member of the assembly, the sensor element being such that inoperation exposure to the particular measurand causes a dimensionalchange of the sensor element in said linear direction, the arrangementof the transducer unit being such that the magnitude of the bend in thefibre is enlarged to an abnormal level which introduces a measurableloss in transmission characteristics of the fibre when the sensorelement is exposed to the particular measurand.
 2. A transducer unit asclaimed in claim 1, wherein the sensor element is made of a materialwhere the dimensional change is reversible on removal from exposure tothe particular measurand, and the sensor element is secured at each endto a least one of the first and second clamp members and to the saidanother clamp member of the assembly.
 3. A transducer unit as claimed inclaim 1, wherein a resilient biasing element is located between thefirst and second clamp members for restoring the position of the centralor second clamp member after activation of the sensor element and in theabsence of the particular measurand.
 4. A transducer unit as claimed inclaim 1, wherein the assembly has three clamp members each of which haslimited movement with respect to the support structure and the threeclamp members are arranged so as to be mutually separated by respectivedistances each of which is less than the length of the correspondingfibre portion when the clamp members are secured to the fibre.
 5. atransducer unit as claimed in claim 1, where the sensor element is anyone of: a) a hydrogel plug for sensing aqueous environments; b) a rubberplug for sensing hydrocarbon presence; c) a wax plug for sensing heat;d) a shape-memory-alloy element for sensing heat.
 6. A transducer unitas claimed in claim 1, including a detection system which enables thepresence of an abnormally bent fibre to be identified.
 7. A method ofdetecting presence of a plurality of measurands; comprising providing alength of optical fibre having a detection system which enables thepresence of an abnormally bent fibre to be identified, and fittingseveral transducer units along the length of the fibre, wherein eachtransducer unit is as claimed in any one of claims 1-4, and whereinseveral different types of sensor elements are used in the transducerunits, the different types of sensor elements being sensitive torespective measurands.